A new technique for constructing a porcelain to metal crown having a fracture resistance comparable to or greater than the fracture resistance to impact forces of the veneer cast metal crown and which overcomes many of the shortcomings of the conventional porcelain jacket crown is disclosed in U.S. Pat. Nos. 4,273,580 and 4,459,112, respectively. In accordance with U.S. Pat. No. 4,273,580, a precious metal foil, preferably a laminate of several precious metal layers, is swaged about a prepared die of a tooth to form a metal matrix upon which a veneering material such as porcelain is fired. However, unlike the conventional porcelain jacket crown, the metal matrix is not removed or separated from the veneering material but is instead retained as a metal coping for the finished porcelain jacket crown. The metal coping is employed as an understructure in the conventional porcelain to metal cast crown.
The physical strength of the metal coping may be substantially enhanced and the ease of preparing the restoration greatly simplified by converting the metal foil starting material into a preformed coping of predetermined geometry as taught and described in U.S. Pat. No. 4,459,112 referred to above. The metal foil starting material is cut into a circular segment and folded to form multiple folds which are uniformly spaced apart and preferably extend radially from a central unfolded area. This multiple fold geometry makes it easy to adapt the preformed coping to the die without the need for superior skill and craftsmanship and even more importantly increases the strength of the coping. Although the preformed coping as above described has certain advantages, it is not essential to the practice of the present invention. In fact, any preformed shape or method of construction may be used.
A metal coping should function to both protect the tooth abutment and as a structural support for the crown or bridge. In the latter respect, the coping supports the veneer material and provides structural strength and rigidity to the dental restoration. An ideal coping will act as an extension of the vital abutment tooth to protect the tooth against fracture and to resist distortion and displacement from the forces applied when chewing food.
The strength of the metal coping after it is swaged and removed from the die is dependent upon its hardness and rigidity. These characteristics may be satisfied using a precious metal which is known to be hard and relatively rigid such as platinum. Rigidity is basically controlled by thickness. Conversely, the ability to adapt and swage the preformed coping to the die so as to assure a proper adaptation with accurate marginal fit requires the coping to be highly workable, i.e., it should be soft and flexible. To be flexible the material should be thin. A dental coping should accordingly be of a material composition which is soft and flexible when it is adapted to the die and yet is hard and rigid after adaptation so as to provide the required structural support for the restoration. These apparent contradictory requirements are met by the coping and crown construction of the present invention. The hardness or softness of a metal is determined by measuring its resistance to permanent indentation. A hardness number is assigned to the material using any one of several conventional hardness tests such as the Vickers hardness test, which uses a diamond pyramid indenter.
In the parent application, U.S. Ser. No. 690,650, the metal coping comprises a low fusing temperature component represented by a gold layer superimposed upon a high fusing temperature component represented by a layer of substantially pure palladium symmetrically disposed between equal layers substantially of gold. It was discovered that this combination of materials will function before sintering as a soft material and after sintering will convert to a harder and more rigid material. It was further discovered that the disposition of the palladium layer between equal layers substantially of gold is essential to increase the fracture resistance of the composite and to minimize any distortion from differences in thermal expansion of the metals during heat treatment.